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dc.contributor.advisorLi, Yanjun
dc.contributor.advisorMarthinsen, Knut
dc.contributor.authorJia, Hailong
dc.date.accessioned2018-09-13T10:48:58Z
dc.date.available2018-09-13T10:48:58Z
dc.date.issued2018
dc.identifier.isbn978-82-326-3069-1
dc.identifier.issn1503-8181
dc.identifier.urihttp://hdl.handle.net/11250/2562479
dc.description.abstractA comprehensive study was carried out to clarify the effects of severe plastic deformation (SPD) and high strain rate deformation on various aluminium alloys and commercial purity titanium. Equal channel angular pressing (ECAP) was conducted on the Al-Cu, Al-Zn and Al-Bi-Zn alloys. Split Hopkinson pressure bar experiments were applied to an Al-Mg alloy and a commercial purity titanium. Detailed analyses were made on the microstructures and mechanical properties, the relationships between which were established as well. An Al-5 wt% Cu alloy supersaturated with Cu in solid solution was subjected to ECAP up to 4 passes (equivalent strain s = 4). The deformation behaviour and the grain structure evolution during ECAP, the strengthening mechanisms of the as-deformed material, and especially, the role played by a high content of Cu have been discussed. The microstructural evolution reveals that a bimodal grain structure composed of coarse micron-sized grains and submicron-sized grains was developed after four passes of ECAP. The bimodal grain structure is caused by several reasons. Firstly, the coarse eutectic Al2Cu particles along grain boundaries can enhance the formation of ultrafine grained (UFG) grains around the particles. Secondly, due to the high solute content of Cu and ECAP via route A, some of the coarse grains are difficult to be refmed into UFG grains. Tensile testing showed that a high ultimate tensile strength of ~500 MPa, a high elongation to failure of ~28% and a uniform elongation of ~5% were achieved simultaneously. High yield strength is a result of a combination of different strengthening mechanisms, i.e. solid solution strengthening by a high content of Cu, strain hardening by a high dislocation density, and grain boundary strengthening. The high solute content of Cu and the bimodal grain structure is supposed to lead to the enhanced rate and capacity of work hardening, respectively, resulting in the improved uniform elongation. In order to clarify the underlying strengthening mechanism of hardening on annealing (HOA) in nanostructured materials prepared by SPD methods, the as-deformed (4P) Al-5 wt% Cu alloy prepared by ECAP was subjected to post-ECAP aging. It showed that a short time aging at low temperatures can increase the ultimate tensile strength and uniform elongation without sacrificing the yield strength. A systematic microstructure characterization by EBSD, TEM and APT has been carried out to investigate the evolution of grain size, dislocation density and solid solution level of Cu as well as the precipitation of Al-Cu precipitates during natural and artificial aging treatments. A quantitative evaluation of different supposed strengthening mechanisms revealed that the segregation of Cu elements at grain boundaries plays a more important role than the grain boundary relaxation and the dislocation source-limited strengthening to compensate the yield strength reduction caused by the decrease in dislocation density and solute content of Cu in solid solution. Al-Bi alloys are of particular interest as potential bearing materials. With the aim to improve their strength, an Al-6Bi-8Zn alloy (in wt%) was subjected to ECAP. At the same time, its microstructural evolution during ECAP was compared with an Al-8Zn alloy to reveal the roles played by the soft Biparticles. After five passes of ECAP (5P), UFG microstructures were obtained in both alloys, while most of the Bi particles were deformed into the flake shapes, which may be good for the lubrication due to higher surface area of Bi particles. The yield strength (YS) of the as-deformed Al-6Bi-8Zn sample was more than three times as that of the as-cast sample. The influence of soft Bi particles on the deformation during ECAP and the final mechanical properties of the Al-6Bi-8Zn alloy has been discussed. It is revealed that soft Bi particles have a strong influence on enhancing grain refmementduring ECAP. At the same time, ECAP is found to accelerate the precipitation of the P(Zn) phase along grain boundaries (GBs). For coarse grained (CG) alloys with high stacking fault energies (SFEs), like aluminium, deformation twins can rarely form. However, we have found that E3{110} incoherent twin boundaries (ITBs) could be generated in a CG Al-8Zn alloy by one pass of ECAP. A systematic investigation shows that the E3{110} ITBs are formed by gradual evolution from geometrically necessary boundaries (GNBs) delineating deformation bands (DBs) by lattice rotation via -twist CSL boundaries. This is a new deformation mechanism in Al alloys, which has never been reported. High strain rate compression deformation (> 103 s"1) by split Hopkinson pressure bar (SHPB) has been conducted on an Al-7 wt% Mg alloy at both room temperature (RT) and liquid nitrogen temperature (LNT). Based on analysis of the microstructural evolution, it is found that conventional coherent twins (CTs) cannot be formed in the Al alloy even at the LNT. The microstructural development is dominated by deformation banding. In addition, the alignment of geometrically necessary boundaries (GNBs) and their boundary planes have been analysed and determined by EBSD. It is found that GNBs delineating deformation bands (DBs) can be formed in grains with compression direction (CD) in the centre of the [100]-[101]-[lll] inverse pole figure. When the misorientation angles of the GNBs are small (< 15°), the GNB planes align with the most slip concentrating {111} planes. Interestingly, with increasing misorientation angles, these GNBs can transformation into S3{112} incoherent twin boundaries (ITBs) via CSL boundaries with higher ∑ values. The deformation structure and deformation behaviour of a commercial purity (CP) Ti alloy subjected to high strain rate (~600 s-1) compression by SHPB have been investigated. Different deformation twins have been observed. It is found that different from the conventional deformation twinning mechanism, a large fraction of the {1121} twin boundaries have gradually transformed from deformation band boundaries with {1121} boundary planes and lower misorientation angles. The mechanism for the formation of the deformation bands and their further transformation into {1121} twin boundaries during SHPB have been investigated experimentally and discussed in terms of Schmid factors of various dislocation slip systems. It shows that the misorientations of the deformation band boundaries increases through accumulative slip of the single basal-(a) dislocations and finally evolves into {1121} twin boundaries.nb_NO
dc.language.isoengnb_NO
dc.publisherNTNUnb_NO
dc.relation.ispartofseriesDoctoral theses at NTNU;2018:139
dc.titleEngineering grain boundary structures of aluminium alloys and titanium by high strain/ strain rate deformationnb_NO
dc.typeDoctoral thesisnb_NO
dc.subject.nsiVDP::Teknologi: 500::Kjemisk teknologi: 560nb_NO
dc.description.localcodeDigital full text not availablenb_NO


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